Nullify a file from sudo

[Trng]

Hi  Team,

is it possible to nullify a file from sudo .i have tried to nullify a file using sudo ,but showing command found /permission denied

sudo >  /var/adm/wtmps

sudo /dev/null /var/adm/wtmps

sudo /dev/null > /var/adm/wtmps

above commands are not working on UNix with sudo ..is it possible ?

rgds,suresh

[Matti_Kurkela]

The problem is that the redirection operators like > are parsed by your current shell before the command is executed.

This means the redirection is done as a regular user, not as root.

• sudo > /var/adm/wtmps

This tells the shell to run "sudo" without any parameters and send the resulting non-error output (if any) to /var/adm/wtmps. If sudo is run without any parameters, it will display an "usage" message as error output, and there will be no non-error output at all. The output redirection fails because your session is not already running as root.

• sudo /dev/null /var/adm/wtmps

This tells sudo to run /dev/null as root, with /var/adm/wtmps as a parameter. Because /dev/null is not an executable program, this will fail without doing anything at all.

• sudo /dev/null > /var/adm/wtmps

This tells the shell to send the non-error output of the "sudo /dev/null" command into /var/adm/wtmps. But because the output redirection happens as a non-root user, the output redirection fails. If the command gets as far as attempting to run "sudo /dev/null", that would also fail like the previous example, because /dev/null is not an executable program.

The most simple & clear way to do this might be:

echo "> /var/adm/wtmps" > /tmp/clear-wtmps.sh
sudo sh /tmp/clear-wtmps.sh
rm /tmp/clear-wtmps.sh

[Trng]

Hi MK,
What is the best way to nullify a file using sudo?

[Patrick Wallek]

You might try:

sudo cat /dev/null > /var/adm/wtmps

[Matti_Kurkela]

(Now I'm again with a broadband network connection, so I have all my usual references available to me, unlike when I gave my previous answer. In particular, I again have convenient access to all HP-UX man pages.)

The problem is two-fold: first, you must quote or escape the ">" so that it will be passed through sudo. Otherwise it will be implemented directly by your current shell using your regular user account, and it will fail.

Second, ">" is not an executable file: it is not even a proper shell built-in command, but a redirection operator.

Sudo is a command that runs executable files as some other user: sudo "> /var/adm/wtmps" would make sudo look for an executable named literally "> /var/adm/wtmps", which obviously does not exist.

When you run a command like "sh <something>", the shell generally expects <something> to be a file, not an arbitrary command line.  (Just like sudo!)

Fortunately, the HP-UX /bin/sh has the -c option (and now I could verify it from the man pages!), which can be used to pass an arbitrary command line to the shell on the command line that is used to start that shell.

So this one-liner will work (I tested it on a HP-UX system):

sudo /bin/sh -c "> /var/adm/wtmps"

This will also work:

echo "> /var/adm/wtmps" | sudo /bin/sh 

This is equivalent to the previous one, but without the explicit "echo" command:

sudo /bin/sh <<EOF
> /var/adm/wtmps
EOF

My original answer can be expected to work on pretty much all normal shells on all Unix-style systems, and is easily understandable and verifiable even if you don't understand the finer details of shell command quoting. This is why I originally called it "the most simple": simplest to understand, although it definitely wasn't the most elegant.